U.S. Pat. No. 5,942,876 corresponding to Japanese Unexamined Patent Publication No. H10-174453 discloses control systems for a three-phase rotary electric machine. A typical example of the control systems is designed to carry out a feedback control for a current to be supplied to each phase of the three-phase rotary electric machine.
In the feedback control, during normal operation of the three-phase rotary electric machine, the control system compares an instantaneous current wave flowing through each phase of the three-phase rotary electric machine in amplitude with an upper limit of a predetermined hysteresis width of a command current wave for each phase and with a lower limit thereof.
The control system of the typical example controls switching timings of each of switching elements of an inverter in bridge configuration based on the comparison result. This allows the instantaneous current wave flowing through each phase to be matched with a request current wave required for the three-phase rotary electric machine to output a request torque.
In contrast, during high RPM of the three-phase rotary electric machine, the control unit can carry out single pulse control.
Specifically, during high RPM of the three-phase rotary electric machine, the control system generates a single voltage pulse in every half cycle of the request current wave to be applied to each of the switching elements.
The feedback control of the control system set forth above may cause the instantaneous current wave of each phase to be deviated from the request current wave for each phase with increase in the RPM of the three-phase rotary electric machine. This may make it difficult for the three-phase rotary electric machine to generate the request torque. This problem will be described hereinafter with reference to FIG. 27.
FIG. 27 schematically illustrates a current wave flowing through one phase of a three-phase rotary electric machine and a duty cycle (switching pattern) of the corresponding one phase. In FIG. 27, a solid line L1 represents an instantaneous current wave flowing through the one phase, and a long and short dashed line L2 represents a command current wave for the one phase. In addition, a chain double-dashed line L3 represents an upper limit of a predetermined hysteresis width of the command current wave, and a chain double-dashed line L4 represents a lower limit of the predetermined hysteresis of the command current wave.
As illustrated in FIG. 27, during the normal operation of the three-phase rotary electric machine with comparative low RPM and magnitude of request torque, the instantaneous current wave L1 follows the command current wave L2 while inching within the range between the upper and lower limits of the predetermined hysteresis width of the command current wave.
During normal operation of the three-phase rotary electric machine, an input voltage of the inverter is sufficiently higher than a back electromotive force created in the three-phase machine. For this reason, the change in velocity of the instantaneous current wave L1 is sufficiently higher than that in velocity of the command current wave during normal operation of the three-phase rotary electric machine. This is a reason why the instantaneous current wave L1 follows the command current wave L2 even though the current wave L1 inches within the range of the predetermined hysteresis width.
In contrast, during high RPM of the three-phase rotary electric machine, the instantaneous current wave L1 is increasingly deviated in phase from the hysteresis width range of the command current wave L2. During high RPM of the three-phase rotary electric machine, an input voltage of the inverter has a narrow deviation compared with a back electromotive force created in the three-phase machine. For this reason, the change in velocity of the instantaneous current wave L1 is substantially equivalent to that in velocity of the command current wave during high RPM operation of the three-phase rotary electric machine. This is a reason why the instantaneous current wave L1 is increasingly deviated in phase from the hysteresis width range of the command current wave L2.
The phase deviation causes on and off timings of each switching element of the inverter to be delayed from those desired for the instantaneous current wave L1 (see FIG. 27), which may cause an output torque of the three-phase rotary electric machine to be deviated from the request torque.
In order to address the torque deviation problem, it is considered to narrow the range between the upper and lower limits of the hysteresis width of the command current wave so as to reduce the deviation between the output torque and the request torque. In addition, it is considered to carry out field-weakening control so as to reduce the electromotive force created in the three-phase machine.
In the former method, however, the reduction in the hysteresis width of the command current wave may cause an increase in the number of switchings of each switching element. The increase in the number of switchings of each switching element may reduce the voltage utilization factor of the three-phase rotary electric machine. Note that the voltage utilization factor is defined as a ratio of an input voltage of the inverter to a single-order component of an RMS value of a line-to-line voltage of a three-phase rotary electric machine.
The reduction in the voltage utilization factor may cause the amplitude of an instantaneous current flowing through each phase of the three-phase rotary electric machine to be reduced, which may increase the torque deviation.
In addition, when the rotor of a three-phase rotary electric machine consists of a permanent magnet, the latter method may degauss the permanent magnet of the rotor.